no one is perfect, least of all me! I am rebuilding a G-9000. In the last 6 months I have rebuilt 2 9090db's, a G-9700, a g-7700, a Pioneer SX-1250, and a Marantz 2270. Plus several others. Well I managed to make a huge error on the G-9000 on the bench now. I am doing a complete recap, and I power up unit with Variac and DBB after each board is rebuilt. First thing one day I wanted to hear it, so I installed PA board which had not been rebuilt yet, but I put them in backwards! Yes, go ahead and laugh at rookie mistake. I powered up using only Variac, no DBB, and at 90 V I could tell there was a problem, then the 15A 3.9 ohm Sandstone resistor on F2808 did its job and fried. No fuses blew, or any other signs of smoke, and unit was powered for maybe 20 seconds. Now recap is finished, 2 PA boards have been examined 9 ways to Sunday by me, and I had 2 friends with Huntron Trackers look at them, and we can find nothing wrong. The SK-97's are fine, etc, but the G-9000 will not come out of protection mode! I have further checked all diodes and transistors on the F2809 Power Supply and Protector board and can find nothing! I am waiting on some new pre-driver transistors to replace the 2 pair of 2SC-1628 AND 2SA-818 because they need it, but should I concentrate on the PA boards or does anyone have any suggestions? Thanks!
SANSUI G-9000 PROTECTION ISSUES AFTER SCREW UP
- Thread starter HankM
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- g-9000 protection
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JoseHH
Super Member
I am no expert, but my limited experience tells me that If your unit does not come out of protection and all the transistors and the differential pair are ok , then it must be that the DC offset is too high. Perhaps your Bias current is also too high? Have you looked into this?. There must be a point where the signal that goes to the protection board can be checked for dc offset before the speaker relay. You may do this with the DBT in place to get it close to zero. Some units do not come out of protection with the DBT if you use a 100W bulb. This happened to me when repairing an AU-D907. But i could still check the DC offset right from the output of the power amplifier board.
Hello! Yes, that is what is weird that I was able to set the bias at the 16mv, starting at only 3mv on both sides, which made me believe they had never been set. Of course I had no voltage at the speakers for the DC offset, so my next plan was to try and set before relays 1 and 2 and see if that does it. By the way, it did come out of protection on my first test w/o the DBB. Some units I work on won't come out of protection w DBB and 100w bulb like you said. taking a break and starting on anther 9090db today! Thanks, Hank Miller!
Not coming out of protect means DC Balance needs to be looked at prior to the relay as you know. You may want to check voltage on the relay contacts to see if you have hi DC. If you cant get to the relay contacts you can check both sides of all the emitter resistors if you can get to them. Anything over 100mv is pointing to an issue. If you find hi DC on one side of one of the resistors and nothing to speak of on the other side, the resistor is probably open, but then you need to check the outputs as emitter resistors usually go bad after an output shorts. Lastly, protect looks at DC balance, which I highlighted above. Protect also looks at a reference voltage from the power supply many times, and at time an over current situation, but overcurrent usually take place when the speaker is shorted or very low impedience. Lastly the protect circuit need its own DC voltages to operate, so look at you regulator supplies and make sure nothing is missing
kevzep
Its all about the Music
You need to check all the regulated supply voltages and verify they are correct.
Significant strain will have been applied by putting the driver boards in backwards, you may have damaged some resistors on the protection/regulated supply board.
If your +/- voltages are not correct, your DC balance will be off and it will be too far out to adjust.
Also, make sure you have the signal input plugged in on the top of the driver boards, if you do not, the unit will not come out of protection as the earth reference for the front end of the driver board is carried by that connector.
You can measure the DC offset on the bypass resistor for the Zobel network
Let us know how you get on...
Significant strain will have been applied by putting the driver boards in backwards, you may have damaged some resistors on the protection/regulated supply board.
If your +/- voltages are not correct, your DC balance will be off and it will be too far out to adjust.
Also, make sure you have the signal input plugged in on the top of the driver boards, if you do not, the unit will not come out of protection as the earth reference for the front end of the driver board is carried by that connector.
You can measure the DC offset on the bypass resistor for the Zobel network
Let us know how you get on...
Verify your fuses are not blown by completely removing each, one by one. Look very carefully for burned up traces on the boards under the driver boards and the driver boards. Maybe the supply boards too.
Great replies, all! Thanks! I took a few days off from the G-9000 and rebuilt another 9090db which went well. Back at the G-9000 today. I replaced the 2 pair of 2sa-818 and 2sc-1628 that won't require a plastic sleeve avoid shorting out! PA boards are 100% and G-9000 came out of protect mode when i switched the Pre-Out and Pre-In switch to seperate which I thought it would. I am getting a bad signal from the Pre-Amp and speakers pop and unit goes into protection mode when switched back to the Connected mode. Signal sounds like an oscilating tone of some sort. Checked all transistors and FET's on the Tone board and all are fine.I am going to scope it now, just wanted to get this thread posted ASAP.
I have been able to confirm that Speaker protection.power supply F-2809 is working flawlessly, main amp come out of protection using pre-amp in and out on seperate setting and sending signal through the input, also confirming that rebuild of PA boards is 100%! In combined mode stays in protection mode, but just discovered unpluging power to the Mic Amp board F-2795 it comes out of protection. Will investigate further but it now appears my stupid blunder did no damage to anything but that 15A 3.9 ohm sandstone which cost $3 to replace. Kind of like the resistor on Pioneer SX-1050 and 1250.
Went back to basics this morning and found that it is the Mic Amp board F2795 causing the protection issues! Poorly designed ribbon cable from mic jack to board shorting out. Unit is finished! I replaced 82 capicitors, a mew record! Actually finished job on shcedule. I will post all of my recent Sansui serial numbers as soon as I get a chance!
kevzep
Its all about the Music
Yes, the ribbon cable had had a previous really bad repair done on it. At least it's high quality cable, and way too long leaving plenty to work with! The 2nd 9090db I had on the bench had the fibre/copper trace ribbon on the connecting the dolby board to the switches that I had to replace, what crap! The 9090db I did last week had the good quality solid wire cable. All in all very happy with this rebuild, especially replacing the the 2sc1628(NPN) and 2sa81(PNP) transistors with KSC2690AS(NPN) and KSA1220AY(PNP) from Mouser. Note: The Pin Out for the new transitors is different, thankfully Sansui added an extra emitter hole on the board making installation of the new transitors easy!